If you are not able to send an email via ATT because you are getting an error. You should try these given steps. Follow these steps one by one and make sure if trouble gets resolved.
1. Refresh the ATT mail account.
2. Sign in your email from another browser.
3. Disable the firewall of your device.
4. Now try to send mail but if you are still having same trouble. You should ask the technical experts via AT&T technical support number 1-800-234-6190 to fix the problem you are facing.